It is true that if a turtle gets stuck on its back, there is the possibility of it dying. In fact, male turtles sometimes fall backwards after copulation and if they are unable to right themselves they can die of starvation.
A turtle’s ability to right itself after falling onto its back also depends upon the particular species. For instance,, the flat shelled turtle that lives in freshwater have long, muscular necks which make it easier for them to flip themselves back over if they happen to accidently get flipped onto their backs.
However for other species of turtles and tortoises, righting themselves can be a task if they have been flipped onto their back. Tortoises and turtles that live on land have much shorter legs and necks which makes it harder for them to get off of their backs. It has been said that early explorers and sailors even used to take giant land turtles with them on long journeys across the oceans as a source of fresh meat, and they would keep the creature upside down so that they weren’t able to wander off of the ship and/or get away.
Some turtles are fortunate enough to have been made equipped with shells that will automatically right themselves if they are flipped over. The very tall domed shells of the star tortoise for instance have a natural tendency to roll back into an upright position. This is also due to the mono-static shape which effectively means that there is one position that they are most stable in. That position being right side up.
For other turtles whose shells are flatter, they have two stable positions on their front and on their back which means that the possibility of getting stuck on their backs is more likely. Even worse are the turtle’s whose shells have even more stable points. Having these stable points makes them more likely to flip onto their backs as well as onto their sides and get stuck halfway through when trying to roll back onto their fronts.
Young turtles that get stuck on their backs in direct sunlight can overheat and die in a very short matter of time. It can even happen in minutes! If a turtle accidently flips onto its back while underwater there is the possibility of it drowning, starving to death or becoming stranded in cold waters and dying from the freezing temperatures.